1. Field of the Invention
This invention relates to a method of electrically cutting a work piece by supplying electric current through a working liquid filling a gap formed between a wire-shaped electrode and a work piece to cut the latter as in the case of a fret saw with control of the cutting width. The invention is intended to provide an electrically cutting method which is effective especially in the so-called "taper-cutting" of a solid material to give a tapered cut surface thereto.
2. Description of the Prior Art
FIG. 1 shows a typical example of the conventional electric discharge type taper-cutting apparatus using a wire-shaped electrode.
A material 1 to be cut is placed on a table 3 which is driven in the directions of the arrows X and Y by an X-axis drive motor 4 and a Y-axis drive motor 5, respectively.
A wire electrode 2 is supplied from a wire supplying reel 7 and is taken up on a wire winding reel 12 through a tension roller 8, a current supplying member 9, an upper wire guide 10 and a lower wire guide 11. An x-axis drive motor 14 and a y-axis drive motor 15 are provided to move the wire guide 10 in the directions of the arrows x and y, respectively, so that the running direction of the wire electrode 2 is changed with respect to the lower wire guide 11. A cutting electric power is supplied by a power supply unit 16 through the work-piece 1, the wire electrode 2 and a current supply member 9. A suitable dielectric liquid (not shown) is continuously supplied through the gap between the wire electrode 2 and the material 1 in the conventional manner. That is, the dielectric liquid flows in the gap between the wire electrode 2 and the work piece 2 at all times during the cutting operation.
A control device 20 is provided to control the X-axis drive motor 4 and the Y-axis drive motor 5 of the table 3 and the x-axis drive motor 14 and the y-axis drive motor 15 of the wire guide 10 so that the work piece 1 is cut into a desired configuration. The control device 20 may comprise a profile control device, an N/C device and a computer.
FIG. 2 shows the case where the work piece 1 is to be cut into a die 1A by the apparatus shown in FIG. 1. The lower surface of the die 1A is a cutting edge, and the upper surface is larger than the lower surface as much as the peripheral portion of the upper surface the width of which is indicated by r. If the material 1 has a thickness t, then the taper angle in cutting the material 1, i.e., the inclination angle .theta. of the wire electrode 2 is: EQU .theta.=tan.sup.-1 r/t (1)
Thus, the wire electrode 2 is inclined through the angle .theta. in a vertical plane perpendicular to the working surface by moving the upper end portion of the wire electrode outwardly. In other words, it is necessary to move the upper wire guide 10 by means of the x-axis drive motor 14 and the y-axis drive motor 15 so that the wire electrode 2 is maintained at the angle .theta. at all times.
In order to maintain this condition, the inclination of the wire electrode 2 is maintained unchanged while the straight portion is cut. However, when the corner portion or curved portion is cut, it is necessary to vary the direction of inclination as the cutting is advanced. When the wire electrode 2 reaches the point B on the upper surface of the work piece 1 and accordingly the point b on the lower surface (the cutting operation effected until the wire electrode 2 reaches the points B and b will be referred to as "a cutting operation in a first cutting mode" when applicable), then the cutting operation of the curved portion will be started (the cutting operation of the curved portion will be referred to as "a cutting operation in a second cutting mode" when applicable). In the cutting of the curved portion, while the locus of the wire electrode is inscribed from the point b to the point c on the lower surface of the material, the locus of the wire electrode must advance from the point B to the point C on the upper surface. FIG. 3 is an enlarged view showing the movement of the wire electrode 2 which is effected in cutting the curved portion. As is apparent from FIG. 3, the wire electrode 2 is moved along the surface of a circular cone having its vertex a. If, in this connection, the rotational radii of the cutting portion of the wire electrode are represented by Rd in the locus on the lower surface of the material 1 and Ru in the locus on the upper surface, then EQU Ru=Rd+r=Rd+t tan .theta. (2)
Accordingly, the relative speed of the wire electrode and the work piece in the upper surface is different from that in the lower surface.
Now, let's consider the width of a groove which is made by the cutting. In the upper surface, the groove width is maintained equal to that in the straight portion up to the point B, because the cutting is carried out substantially with the upper limit of the cutting capability similar as in the straight cutting operation. On the other hand, in the lower surface, the relative speed becomes lower than that in the straight cutting operation, and accordingly the groove width is increased as shown in FIG. 4.
Especially in the case where a work piece is cut into a die as described above, the accuracy of its cutting edge is lowered. This is one of the serious disadvantages accompanying the conventional electrically taper-cutting method.